In recent years, ceramic superconductors have been developed which retain their superconducting properties at relatively high transition temperatures (T.sub.c), i.e. at temperatures above about twenty degrees absolute or 20 Kelvins. Consequently, although modern ceramic superconductors still need to be cooled to be operational, they do not need to be cooled to as low a temperature as the earlier superconductors. The new superconductors can therefore be used in a large number of applications which have not heretofore been economically feasible.
Not surprisingly, many of the potential applications for high-T.sub.c ceramic superconductors require forming the superconductor into a wire configuration. Indeed, wire configurations for superconductor materials have been proposed. For example, a superconductor wire configuration is disclosed in pending application Ser. No. 265,827, now U.S. Pat. No. 5,047,38 for an invention entitled "Substrate for Ceramic Superconductor", which is assigned to the same assignee as the present invention. These wires, however, can be damaged in numerous ways. Thus, to protect the ceramic superconductor wire from contamination by impurities which could adversely affect the superconducting properties of the ceramic, methods have been developed for encasing the superconductor wire in a protective cladding Ideally, the protective cladding is itself electrically conductive, in order to enhance the electrical conductivity of the superconductor wire as well as protect the wire. One method for encasing a superconductor wire in a protective cladding is disclosed in co-pending U.S. patent application for an invention entitled "Anhydrous Electrophoretic Silver Coating Technique" which is assigned to the same assignee as the present invention.
Silver is ordinarily the preferred protective cladding material when the superconductor material is a ceramic, because silver will not readily react with the ceramic superconductor material. This is important because any reaction or interdiffusion of material between the protective cladding and the ceramic superconductor will adversely affect the superconducting properties of the superconductor. Additionally, silver is preferred because silver can establish an effective protective cladding which substantially blocks the diffusion of water, carbon dioxide, and other environmental impurities through the silver cladding and into the ceramic superconductor. Also, silver can form a low contact resistance electrical contact to the high temperature superconductors as well as to normally conductive material such as copper or solder, normally used for electrical current transport. Thus, silver is an ideal bridging material between the superconductor and other conductive materials.
Silver does not, however, prevent the diffusion of oxygen through the protective cladding into the ceramic superconductor. This is desirable, because such oxygen diffusion is necessary in order to properly establish the oxygen content of the superconductor incident to the manufacturing process. To illustrate why it is important to properly establish the oxygen content of the superconductor, consider the familiar 1-2-3 ceramic superconductor which has the chemical formula YBa.sub.2 Cu.sub.3 O.sub.7-x, where x has values from zero to one (0-1). For this superconductor, in order to effectively operate as a superconductor at as high a temperature as possible, it is necessary that the superconductor be oxidized after the manufacturing process to establish a value for x that is as close to zero as possible. Specifically, the transition temperature for the 1-2-3 superconductor is about ninety Kelvins (90K) when x is zero, but is substantially reduced when x is greater than 0.3. Accordingly, ceramic 1-2-3 superconductors, including those which have a silver coating, are typically heated for a relatively lengthy period, e.g., several days, to establish an optimum oxygen concentration in the superconductor.
In the manufacture of production lengths of superconductor wire, it will be appreciated that the wire must ordinarily be wound onto a spool to facilitate the economic oxidation of an entire length of wire in a single furnace. When these relatively long production lengths of superconductor wire are wound onto a spool, however, portions of the wire can overlap or contact other portions of the wire. Unfortunately, when the spool of wire is heated in the furnace, e.g., to oxygenate the superconductor in the case of 1-2-3 superconductor, or to sinter the superconductor at elevated temperatures in the case of bismuth-based superconductors, this overlapping contact between portions of the wire allows the silver cladding material from one portion of the wire to diffuse into the silver cladding of other portions of the wire with which it is in contact. This diffusion of silver can not only impair the protective characteristics of the silver cladding, but also prevent an effective unspooling of the oxygenated wire because adjacent portions of the silver cladding can stick together. The present invention recognizes that a method can be provided to substantially prevent the diffusion of material between portions of a silver cladding which are in contact with each other during the superconductor oxidation process.
Accordingly, it is an object of the present invention to provide a method and apparatus for heat processing a silver clad superconductor wire which substantially prevents the diffusion of material from the silver cladding of the wire into adjacent segments of silver cladding. It is a further object of the present invention to provide a method and apparatus for heat processing a superconductor wire which permits the wire to be wound onto a spool in juxtaposed coils incident to oxidizing the superconductor. Another object of the present invention is to provide a method and apparatus for heat processing a superconductor wire which is relatively easy and cost-effective to use. A further object of the present invention is to provide a method for heat treating silver clad spooled superconductor wire which will result in wires which are not stuck together and not broken so that the wires may be unspooled as long continuous lengths of high quality superconductor wire.